Driving control of a reciprocating CPR apparatus

ABSTRACT

The disclosed mechanical cardio-pulmonary resuscitation (CPR) apparatuses, systems, and devices have a plunger and a plunger displace sensor that can sense plunger displacement information during reciprocating cycles of the plunger. The disclosure CPR apparatuses, systems, and devices also have a microprocessor unit that can receive sensed plunger displacement information from the sensor and generate plunger driving instructions based on the plunger displacement information. The plunger driving instructions have one or both of a plunger driving force and a plunger amplitude for the reciprocating cycles.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/549,164, filed Jul. 13, 2012, which is a division of U.S. patentapplication Ser. No. 12/523,082, filed Aug. 13, 2009, now abandoned,which is a national stage entry of International Patent Application No.PCT/SE2008/000022, filed Jan. 14, 2008, which claims the benefit ofSwedish Application No. 0700094-6, filed Jan. 18, 2007.

BACKGROUND

In cardio-pulmonary resuscitation (CPR) repeated compressions areadministered by hand or by apparatus to the chest of the person beingresuscitated to maintain circulation and oxygenation of blood.Concomitant with the compressions electrical shocks can be provided tothe patient to make the heart beat again. Gas-driven reciprocating CPRapparatuses have been known in the art and used in practice for a longtime. Providing compressions of correct depth is an important factor forsuccess of the method.

SUMMARY

The disclosed mechanical cardio-pulmonary resuscitation (CPR)apparatuses, systems, and devices have a plunger and a plunger displacesensor that can sense plunger displacement information duringreciprocating cycles of the plunger. The disclosure CPR apparatuses,systems, and devices also have a microprocessor unit that can receivesensed plunger displacement information from the sensor and generateplunger driving instructions based on the plunger displacementinformation. The plunger driving instructions have one or both of aplunger driving force and a plunger amplitude for the reciprocatingcycles.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained by reference to preferred but notlimiting embodiments thereof illustrated in a rough drawing, in which

FIG. 1A is a sectional view of a first embodiment of the apparatus ofthe invention disposed on the chest of a patient shown in a transversesection at the level of the eight thoracic vertebra (T8) and viewed in acranial direction;

FIG. 1B is a detail view of the embodiment of FIG. 1A, in the same viewand enlarged;

FIG. 1C is a section A-A (FIG. 1A) through a modification of theapparatus of FIG. 1A;

FIG. 1D is a detail view of another modification of the apparatus ofFIG. 1A showing solenoid valve control unit with a pair of solenoidvalves;

FIGS. 2A-2H show the embodiment of FIG. 1A and in the same view, inconsecutive states of chest compression by reciprocating displacement ofits piston and compressing pad;

FIG. 3 is a block scheme of a solenoid valve control program;

FIGS. 4A-4D show another embodiment of the apparatus of the invention,in the same view as in FIG. 1A and, in FIGS. 4C and 4D, partiallyenlarged;

FIG. 5 shows a further embodiment of the apparatus of the invention, inthe same view as in FIG. 1A;

FIGS. 6A-6C are graphs illustrating the effect of driving gas valveopening times on gas consumption in reaching and maintaining a desiredcompression depth against a given resilient force in CPR modelexperiments;

FIG. 7 is a rough sketch of the compression testing apparatus used inthe experiments illustrated in FIGS. 6A-6C.

DETAILED DESCRIPTION

In the following, “Compression Depth” signifies the maximum sternaldeflection during a compression/decompression cycle. An appropriateCompression Depth for adult persons corresponds to a sternal deflectionof 20%; the compression depth for a chest with an anterior-posteriordiameter of 25 cm thus is 5 cm. In contrast “compression depth” in thefollowing refers to a sternal deflection during acompression/decompression cycle smaller than the maximum deflection orto sternal deflection in general.

Shallow compressions may be insufficient to restore circulation andoxygenation while compressions that are too deep may damage the ribs andthe soft tissues of the chest. There is thus an optimal CompressionDepth or a narrow range of optimal Compression Depths. Administration ofcompressions of optimal Compression Depth may be controlled byadministering compressions of a given force. Alternatively, a desiredCompression Depth may be set by an operator; it may be optionallychanged during resuscitation. Alternatively, the Compression Depth in aCPR apparatus can be set by limiting the stroke of the piston in theapparatus to the average optimal Compression Depth for an adult person.A given compression force results in a compression to a CompressionDepth at which the compression force is balanced by the resistive forceof the chest tissues. Since even adult persons differ in their chestanatomy a given compression force may result in compression of varyingdepth in a group of persons. Therefore the direct determination ofCompression Depth during cardiopulmonary resuscitation and its use forcontrol of the apparatus by which the compressions are administered isdesirable.

The determination of compression depths during cardiopulmonaryresuscitation is known in the art. An accelerometer-based compressionmonitor is placed on the patient's sternum, the arm of a rescueradministrating manual heart compressions or on acompression-administrating part of an automatic CPR device. The chest isthen compressed. The accelerometer signal is integrated and fed to aprocessor, which calculates the compression depth from the signal by useof complex algorithms. The accelerometer is electrically connected tothe processor.

In the administration of repeated compressions in cardiopulmonaryresuscitation, the use of apparatus based on a reciprocating pistonprovided with a chest compression pad and mounted in a cylinder isknown. The piston is driven by a compressed gas. The Compression Depthadministered with such an apparatus is limited by physical meanscomprised by the apparatus and set from start to from about 40 mm toabout 50 mm for an adult person.

The embodiment of the apparatus 1 of the invention shown in FIGS. 1A-2Hcomprises a cylinder housing of a diamagnetic material having a sidewall 2, a bottom 3 and a top wall 4. A piston 5 with a circumferentialsealing 9 is mounted in the housing and defines an upper compartment Aand a lower compartment B. A plunger 6 extends downwards from the centerof the piston 5, passing through a central bore in the bottom 3 of thehousing. At its free end the plunger 6 carries a chest compression pad 7provided with a flexible circumferential lip 8. The piston 5/plunger6/compression pad 7 is mounted displaceably in the cylinder housing. Aneodymium magnet ring 14 is mounted at the lower face of the piston 5with its south pole S facing the side wall 2. An array of unipolarHall-Effect digital switches (“unipolar Hall switches”) 15, 16, 17, 18,19 is mounted at the outer wall of the cylinder 1 in an axial direction.The unipolar Hall switches 15, 16, 17, 18, 19 are characterized by theirmagnetic operating threshold. If the Hall cell of a switch 15, 16, 17,18, 19 is exposed to the magnetic field of the south pole exceeding theoperating threshold, the output transistor is switched on. If the fielddrops below the switching threshold, the transistor is switched off. Onepole of each of the unipolar Hall switches 15, 16, 17, 18, 19 isgrounded at 23 whereas the other is fed with 3 V DC by lines 15′, 16′,17′, 18′, 19′, respectively, connected to a microprocessor unit 13. InFIG. 1B the field lines 24 of the magnet's 14 south pole S are shown inrespect of unipolar Hall switches 18, 19 to illustrate how the latterare influenced during a displacement of the plunger 6. The effect (Halleffect) by which the switches 15, 16, 17, 18, 19 are closed by theinfluence of the field of the magnet 14 allows to monitor the passage ofthe plunger by the microprocessor unit 13 (FIG. 3). During passage ofthe magnet 14 the circuit of the respective switch 15, 16, 17, 18 or 19is closed, the current passing a closed switch being recorded by themicroprocessor unit 13. After passage of the magnetic field therespective switch is again opened except for if the plunger stops in aposition in which the magnetic field does cover it after stop. Thisallows the microprocessor unit 13 to keep track of the movement of thepiston 5/plunger 6/pad 7 assembly and, in particular, its position atthe end of its downward or, less important, upward movement, and tocontrol the provision of compressed breathing gas to compartment A bythe solenoid valve based on that position.

For reasons of simplicity number of unipolar Hall switches in theembodiment of FIGS. 1A to 2H is confined to five. An embodiment thatallows to obtain fine tuning of positional control can comprise a highernumber of unipolar Hall switches and/or have the switches disposed inthe region of the compression depth level where the determination ofposition of piston 5/plunger 6/compression pad 7 is most important.Other Hall-effect switches like bipolar and omnipolar Hall-effectswitches may be used for sending the field of magnet 14.

In a modification of the embodiment of FIG. 1A the ring magnet 14 isexchanged for a rod magnet 6′ (FIG. 1C). For reasons of balance acounter weight 27 is mounted diametrically opposite to the magnet 6′ atthe lower face of the piston 5′. The use of a rod magnet 6′ requires thearrangement of a means preventing rotation of the piston 5′. In FIG. 1Cthe rotation preventing means comprises two diametrically oppositeaxially extending flanges 28 protruding from inner face of the cylinderside wall 2′ and co-operating with diametrically opposite axiallyextending slits in the side wall of piston 5′. Also shown is aHall-effect switch 18′ mounted at the outer face of the side wall 3′opposite to the south pole of magnet 14′.

Returning to the embodiment of FIGS. 1 and 2A-2H the upper compartment Aof the housing is defined by the top face of the piston 5, a firstportion of the side wall 2 of the housing, and the top wall 4 of thehousing, whereas its lower compartment B is defined by the bottom faceof the piston 5, the bottom face of the magnet 14, a second portion ofthe side wall 2 and the bottom wall 3. An opening 22 in the bottom wall3 allows air to enter into compartment B or to be expelled from itdepending on the direction of displacement of the piston 5.

A tube 10 for providing compressed breathing gas from a gas supply suchas a gas cylinder or other container of compressed breathing gas (notshown) is mounted at and communicates with an opening 25 in the top wall4. Near the opening 25 a venting tube 21 branches off from tube 10. Tube21 can be put in communication with a breathing mask (not shown) borneby the patient under cardiopulmonary resuscitation. A three-way solenoidvalve 11 controlled by a solenoid control unit 12 is mounted in thelumen of tube 10 at the branching of the venting tube 21. In a firstposition P1 the solenoid valve 11 allows compressed breathing gas toenter compartment A through opening 25. In a second position P2 thesolenoid valve 11 allows to vent compressed air in compartment A throughventing tube 22. The solenoid valve 11 is only shown schematically inthe Figures; its design allows switching between positions P1 and P2without passing an intermediate position in which the lumina of tubes 10and 21 and the compartment A are in simultaneous communication. Thesolenoid valve is actuated by a solenoid valve control unit 12 receivingactuation signals from the microprocessor unit 13 via line 20. Themicroprocessor unit 13 and the solenoid valve control unit 12 areenergized by a dry battery (not shown). The three-way solenoid valve 11of the embodiment of FIGS. 1 and 2A to 2H can be exchanged for a pair ofsolenoid valves 11′, 11″ actuated by a solenoid valve control unit 12′(FIG. 1D). Reference numbers 4′, 10′, 21′ and 25′ identify elementscorresponding to elements 4, 10, 21 and 25, respectively, of theembodiment of FIGS. 1 to 2H.

After leaving the gas cylinder the compressed breathing gas isdecompressed in controlled manner (not shown) to a working pressure,which is kept about constant during CPR. The gas of working pressure issuitable held in a reservoir from which the gas of working pressure isadduced to the compartment A via tube 10 so as to provide it at an aboutconstant gas pressure over time. This allows the provision of acontrolled compression force via the piston 5, the plunger 6, and thepad 7 to the chest of a patient. Since the adduction of compressed gasthrough tube 10 and the build-up of gas pressure in compartment A is adynamic process governed by the pressure of the gas in the gasreservoir, the gas pressure in the compartment 10 (the pressure of the“provided” driving gas) is not in equilibrium with the pressure of thedriving gas at the source over an initial portion of the compressionphase.

The compression pad 7 is loosely placed on the chest 30 of a person tobe provided chest compressions (FIG. 1A). The person is in a recumbentposition with the pad 7 placed on the skin 31 above the sternum 32.Reference numbers denote: 33, right ventricle; 34, left ventricle; 35,esophagus; 36, descending aorta; 37, body of the eight thoracic vertebra(T8); 38, spinal cord; 39 left arc of ribs.

The function of the apparatus of the invention will now be explainedwith reference to FIGS. 2A through 2H.

The position at the start of dispensation of chest compression is shownin FIG. 2A, which corresponds to FIG. 1A except for that only the skin31 of the patient's chest in the sternal region is shown. In the Figuresthe uncompressed level of the skin 31 at the application site of the pad7 is designated 0. The solenoid valve 11 is in the venting position P1and the plunger is in an unloaded state. By opening of the venting valve11 (valve position P2) compressed air is made to flow from the gascylinder to tube 10 and to enter compartment A through opening 25. Theincreasing air pressure in compartment A starts to force the piston 5downwards in the direction of bottom wall 3 (FIG. 2B; start of downwardmovement of piston 5 indicated by an arrow). At start the south pole Sof the magnet 14 is disposed between Hall switches 15 and 16. During itsdownward movement (FIG. 2C; valve 11 in position P2; skin level atintermediate position P during downward movement) the south pole S ofthe magnet 14 passes Hall switches 16, 17 and stops at the level ofswitch 18 (skin level Rat initial full compression=Compression Depth inan initial cycle of CPR). It stops at because, at this level, thecompression force of the compressed gas acting on the piston 5transferred by the pad 7 to the patient is balanced by the resilientcounterforce of the compressed chest tissues. As explained above, themicroprocessor unit 13 keeps track of the position of the piston 5during its downward (and upward, if desired) movement, and recognizesthe exact moment at which the piston 5/plunger 6/rod 7 assembly hasreached its extreme position during its downward movement (FIG. 2D,indicating the infinitesimal last downward movement of piston 5/plunger6/rod 7 assembly prior to stop with the valve 11 in position P1; FIG.2E, the moment of stop with the valve 11 in position P1; and, at thesame moment, FIG. 2F, an immediate switch of the valve 13 from positionP1 to P2. As explained above and if desired, the switch of the solenoidvalve 11 from P1 to P2 can be made to occur slightly earlier, that is,prior to the piston 5/plunger 6/rod 7 assembly reaching its downwardstop position by programming of the microprocessor 13 correspondingly.The recognition of the time when the piston 5/plunger 6/rod 7 assemblyreaches its lower end or bottom position moment thus is used for controlof the solenoid valve so as to switch it from position P2 to P1. Theflow of compressed gas into compartment A is stopped at the moment wherethe piston 5/plunger 6/rod 7 assembly reaches the desired extremeposition (Compression Depth) or slightly before that moment. Thereby theprovision of driving gas is optimized and thus economized. This is ofparticular importance for a CPR apparatus to be used outside facilitieslike a hospital where practically unlimited resources of compressed gasof various kind are available. In the state of the apparatus shown inFIG. 2F compartment A is vented via tube 21. If the driving gas is abreathing gas the vented gas or a portion thereof, which is still of aslightly higher pressure than ambient air, can be adduced to thepatient's lungs via a breathing mask or by intubation (not shown). Theventing of compartment A stops the load on the piston 5/plunger 6/rod 7assembly (FIG. 2G) and thus the compression of the patient's chest. Theresilient nature of the chest makes it expand and push the piston5/plunger 6/rod 7 assembly back to its start position (FIG. 2H).

The microprocessor unit 13 of the apparatus of the invention isprogrammed in a manner so as to sample and store positional data overone or several cycles, and to use such data for control of a latercycle.

For reasons of simplicity and to better illustrate the principles of theinvention the apparatus the invention shown in FIGS. 1 to 2H has beensimplified in respect to commercially available apparatus of this kindin regard of ancillary features. Thus, the upward movement of the piston5/plunger 6/rod 7 assembly of this embodiment of the apparatus of theinvention is passive, that is, driven by the resilient force of thepatient's chest, whereas it can be advantageously be driven by means ofthe compressed breathing gas used in the apparatus. In such case asubstantially more complex arrangement of valves and gas lines foradducing compressed gas to compartment B and venting it from there isrequired. The provision of and additional pressure means for actively,that is, substantially independent of the resilient forces of acompressed chest, returning the piston to its start position doeshowever not change the principles of the present invention, which evenmight be used to optimize the use of compressed gas in the displacementof the piston 5/plunger 6/rod 7 assembly in such upward (decompressing)direction.

In a second embodiment of the CPR apparatus of the invention 101 shownin FIGS. 4A to 4D the position of the piston 105 and thus thecompression depth is determined by means of a source of visible light114 and a number of photo detectors 143, 144, 145, 146, 147, 148, 149,all disposed in a radial direction, with the light source 114 innermost,on the inner face of bottom wall 103. The light source 114 is a redlight photodiode whereas the photo detectors 113-119 are silicon basedphotodiodes operated in photoconductive mode.

The narrow and substantially parallel beam of light 124′ of thephotodiode 114 is directed at the lower face of the piston 114, which isprovided with a ring mirror 130, at an angle .alpha. and in the same aradial direction in respect of the piston 105 axis as that of thedisposition of photo detectors 113-119. The incident beam 124′ isreflected at the same angle .alpha. in the direction of photo detectors113-119 disposed on the bottom 103. The distance between the inner faceof the bottom 103 and the lower face of the piston 105 provided with themirror element 130 determines which of the photo detectors 143-149 ishit by the reflected beam 124″. In a position of the piston 105 near thebottom wall 103 (distance d1, FIG. 4C) the reflected beam 124″ hits thenext but innermost photo detector 148, whereas in a position of thepiston near the top wall 104 (distance d2, FIG. 4D) the next butoutermost photo detector 144 is hit. During a downward movement of thepiston 105 the reflected beam 124″ thus will sweep, depending on itsstart position and its end position (Compression Depth position) overall or only some of the photo detectors in a radially inward direction.The photo diode 114 and the photo detectors 143, 144, 145, 146, 147,148, 149 are connected to a microprocessor unit 113 via separateconductors 114′, 143′, 144′, 145′, 146′, 147′, 148′, 149′, respectively,which are bundled in a cable 131. The microprocessor 113 uses thesignals from the photo detectors 143-149 in a time frame to control gasflow in the apparatus 100 by a solenoid valve 111 operated by a solenoidcontrol unit 112 in a manner corresponding to that described in Example1 for the electric signals generated by the Hall-effect switches 15, 16,17, 18, 19. In FIGS. 4A-4D reference numbers 106, 110, 120, 121 refer toa plunger 106 carrying a chest compression pad (not shown), a tube 110for adducing compressed breathing gas, to an electrical connection 120between the microprocessor unit 112 and the solenoid control unit 112,and to a tube 121 for venting compressed breathing gas used fordisplacement of the piston 105, respectively.

In a third embodiment of the invention shown in FIG. 5 the stroke of thepiston 305 is limited by an annular stop 320. At its lower extremeposition the piston 305 hits and thereby closes a contact switch 315 ofan electrical circuit comprised by a microprocessor unit 313. Themicroprocessor unit 313 thereby receives information about the moment atwhich the Compression Depth is reached. Based on this information themicroprocessor's 313 issues a closing command to the control unit 312 ofsolenoid valve 311, in particular in a following cycle prior to theexpected time of contact. In this embodiment the provision of a gasinlet tube 340 to provide driving gas to the closed lower chamber B ofthe cylinder housing 302, 303, 304 illustrates the principle of assistedpiston 305 return that can be applied to all embodiments of theinvention, if desired.

Example 2

Solenoid Valve Control Program

In the following an example of a simple main valve control program isprovided (Table 1). In the example consideration is given to one Halleffect element (Hall switch), which is placed at about a desired levelof piston 5/plunger 6/rod 7 assembly stop (bottom level). Time open forthe decompression main valve is set to 300 ms; while this parameter isfixed in the Example, it could be controlled in precisely the same wayas time open for the compression main valve.

TABLE 1 Initialize set t_open = 300 [ms] set adjust = true (Parallelprocess #1, controls main valves) While true do Is_down = falseMain_valve_comp = true /opens compression main valve Wait   t_open/holds main valve open for t_open ms Main_valve_comp = false /closescompression main valve Wait 300 − t_open /wait the rest of thecompression Phase Main_valve_decomp = true /opens decompression mainvalve Wait   300 /waits until whole cycle is completeMain_valve_decomp=false /closes decompression main valve If adjust =true If is_down = true T_open = t_open − 20 /decreases t_open ElseT_open = t_open+10 /increases t_open Adjust=false /adjustment is nowComplete End if End while (Parallel process #2, sampleshall_element_signal input and updates variable “is_down”) while adjust =true do hall_effect_sample =read_digital_input_signal_of_hall_effect_element is_down = is_down orhall_effect_sample /true if piston has reached end while /hall elementduring cycle

When the program starts the program variable (t_open), which controlsthe time the air supply port for the compression phase is open, is setto 300 ms, which is the maximum possible value. The apparatus thenperforms one cycle (compression and decompression) with this setting.During the cycle the signal from the Hall effect element is sampled. Ifthe piston reaches the bottom of the cylinder it will be registered bythe Hall effect sensor signal as a high voltage, sampled by the function“read digital input signal_of_hall_effect_element” and then written tothe variable is_down. is_down is the variable that indicates whether thepiston has reached its bottom position during the cycle, and thendetermines which adjustment of t_open shall be performed. If a triggerwas detected (and is_down set to true), than the variable t_open islowered by 20 ms. This is repeated for every cycle until there is notrigger detected. During this last cycle the piston is likely to havestopped just before it reached the Hall effect element, such as a fewmillimetres from demand position. As is_down now is false the variablet_open is increased by 10 ms, which makes the piston move a little bitfurther down next cycle; this is then considered to be the finalposition at which the update procedure stops (since the variable adjustis set to false it cannot become true again). This setting will be usedfor the rest of the treatment or may be changed after some time such as,for instance, 10 minutes from start, to adapt the compression to theaforementioned change in physical properties of the chest. A blockdiagram of the program is shown in FIG. 3.

Example 3

The effect of the method of the invention in the control of compresseddriving gas is demonstrated by three experiments illustrated in FIGS.6A-6C. The experiments were carried out with an air-driven reciprocatingCPR device mounted on a test bench. The CPR apparatus comprises acompression cylinder 208 comprising an upper compartment 219 and a lowercompartment 220 delimited in respect of each other by a piston 216arranged displaceably in the cylinder 208. The apparatus furthercomprises a breast compression pad 210 attached to the piston 216 via ashaft 211, a valve control unit 212 with a valve manifold, and a gasline 213 supplying driving gas from source of compressed gas (not shown)to the compression cylinder via the valve control unit 212. The stroke(str) of the piston 216 is limited to 55 mm by means of upper 217 andlower 218 stroke limiters disposed in the upper and lower compartments,219, 220, respectively. The gas pressure in the upper compartment 219 ismeasured by a manometer 214. The test bench comprises a flat base 201 onwhich the CPR apparatus is mounted via a pair of legs 209. Thecompression pad 210 abuts a top face of a sternal plate 204 resting on asupport 202 via an interposed force sensor 203. The support 202 restsdisplaceable in a vertical direction on the base 201 via compressioncoil means 205, which mimic a resilient chest. A linear sliding rail 206fixed at the base 201 allows to read the position of the sternal plate204 and the compression pad 210 by means of a linear slide guide 207running on the rail 206. The slide guide 207 comprises a positionsensor. As indicated by reference numbers 203′, 207′, 212′ and 214′signals from the force sensor 203, the position sensor 207, the valvecontrol unit 212 and the manometer 214 are electrically transferred to acontrol unit 215 in which they are stored and from which the can berecalled and displayed. The resisting force of the compression coilmeans 205 against further compression at a Compression Depth of about 50mm was set to about 500 N (FIG. 6A, 477 N) or half of that value, about250 N (FIG. 6B, 239 N; FIG. 6C, 230 N). These and other parameter valuesare listed in Table 2. In all experiments the pressure of the drivinggas fed to the compression cylinder was 2.91 bar.

TABLE 2 Max. cylinder Compr. Decompr. Inlet Compr. Reciprocatingpressure, phase, phase, valve depth, Exp. # frequ., 1/min bar Force (N)sec sec open, sec mm a 98 2.91 477 0.31 0.30 0.31 52.1 b 99 2.88 2390.31 0.30 0.31 56.0 c 99 1.36 230 0.30 0.31 0.09 55.5

Experiment (a), FIG. 6A, reflects the situation at the start of CPR,that is, during the first compressions administered to a patient. Toprovide optimal treatment it is necessary to obtain full compression,that is, a Compression Depth of about 50 mm for the average adultperson, right from start and at an adequate rate of about 100compressions per minute and even more. For this reason the minimumpressure of the driving gas is set to about 3 bar or more. This sufficesto develop a compression force of about 500 (477 N in the experiment),by which the compression coil means 205 are compressed to a full stroke(str, FIG. 7), the lower stroke limiter 218 being reached at point L inFIG. 6A.

Experiment (b), FIG. 6B, reflects the situation after provision ofcompressions to a patient for a few minutes. During this time period theresistance of the chest diminishes by about 50%. The force ofcompression necessary to obtain a desired Compression Depth of about 50mm thus is substantially reduced. In this experiment the resistance ofthe compression coil means 205 is set to about half (239 N) of theresistance in experiment (a). A compression profile similar to that ofexperiment (a) is obtained, except for the downward stroke occurringconsiderably faster, the lower stroke limiter 218 being reached at M. Inboth experiments (a) and (b) the driving gas inlet valve is open duringthe entire compression phase. It is closed simultaneously with theopening of the venting valve, by which the pressure in the compressionchamber is released to allow the piston 216 and the compression pad 204return to their starting position defined by upper stroke limiter 217.This return movement is supported by means of driving gas being fed to alower chamber 219 in the housing (FIG. 7).

In experiment (c), FIG. 6C, substantially the same time v, compressionpad displacement curve is obtained as in experiment (b). Experiment (c)differs from experiment (b) only in that the valve by which driving gasis adduced to the upper chamber 219 is kept open for a comparativelyshort time only. It is made to close at N (FIG. 7) even before theCompression Depth is reached. The final or maximum gas pressure in thecompression compartment is thereby limited to about half of the pressurein experiment (b), and a corresponding saving of driving gas isobtained.

Experiments (a) to (c) demonstrate that up to 60 percent and even up toabout 70 percent of driving gas can be saved by the method and theapparatus of the invention. This has been confirmed in in-vivoexperiments in a pig model.

What is claimed is:
 1. A mechanical cardio-pulmonary resuscitation (CPR)apparatus, comprising: a plunger; a plunger displacement sensorconfigured to sense plunger displacement information during a first setof reciprocating cycles; a microprocessor unit configured to receive thesensed plunger displacement information from the plunger displacementsensor and to generate plunger driving instructions for administering asecond set of reciprocating cycles based at least in part on the sensedplunger displacement information, the generated plunger drivinginstructions including one or both of a plunger driving force and aplunger displacement amplitude.
 2. The CPR apparatus of claim 1, whereinthe plunger is mechanically-controlled.
 3. The CPR apparatus of claim 1,wherein the first set of reciprocating cycles is an initial set ofreciprocating cycles administered at an initiation of CPR treatment andthe second set of reciprocating cycles is a subsequent set ofreciprocating cycles timed after the initial set.
 4. The CPR apparatusof claim 1, further comprising a memory configured to store the sensedplunger displacement information.
 5. A mechanical cardio-pulmonaryresuscitation (CPR) apparatus, comprising: a plunger; a plungerdisplacement sensor configured to sense plunger displacement informationduring reciprocating cycles and a current position of the plunger; amicroprocessor unit configured to receive the sensed plungerdisplacement information and the current position of the plunger fromthe plunger displacement sensor and to generate plunger drivinginstructions based at least in part on the sensed plunger displacementinformation and the current position of the plunger, the generatedplunger driving instructions including one or both of a plunger drivingforce and a plunger displacement amplitude.
 6. The CPR apparatus ofclaim 5, further comprising a memory configured to store the sensedplunger displacement information.
 7. The CPR apparatus of claim 6,wherein the memory is further configured to store one or both of thecurrent position of the plunger and the generated plunger drivinginstructions.
 8. A mechanical cardio-pulmonary resuscitation (CPR)apparatus, comprising: a plunger having a reciprocating part and anon-reciprocating part; a plunger displacement sensor mounted on thenon-reciprocating part of the plunger, the plunger displacement sensorconfigured to sense plunger displacement information related todisplacement of the reciprocating part with respect to thenon-reciprocating part during a first set of reciprocating cycles of theplunger; and a microprocessor unit configured to receive the sensedplunger displacement information from the plunger displacement sensorand to generate plunger driving instructions for administering a secondset of reciprocating cycles based at least in part on the sensed plungerdisplacement information, the generated plunger driving instructionsincluding a plunger driving force and a plunger displacement amplitude.9. The CPR apparatus of claim 8, wherein the plunger is gas-driven. 10.The CPR apparatus of claim 8, further comprising a memory configured tostore the sensed plunger displacement information.
 11. The CPR apparatusof claim 8, wherein the first set of reciprocating cycles is an initialset of reciprocating cycles administered at an initiation of CPRtreatment and the second set of reciprocating cycles is a subsequent setof reciprocating cycles timed after the initial set.
 12. A mechanicalcardio-pulmonary resuscitation (CPR) apparatus, comprising: a plungerhaving a reciprocating part and a non-reciprocating part; a plungerdisplacement sensor mounted on the non-reciprocating part of theplunger, the plunger displacement sensor configured to sense plungerdisplacement information related to displacement of the reciprocatingpart with respect to the non-reciprocating part during reciprocatingcycles of the plunger, the plunger displacement sensor furtherconfigured to sense a current position of the reciprocating part of theplunger; and a microprocessor unit configured to receive the sensedplunger displacement information from the plunger displacement sensorand to generate plunger driving instructions based at least in part onthe sensed plunger displacement information and the current position ofthe reciprocating part of the plunger, the generated plunger drivinginstructions including a plunger driving force and a plungerdisplacement amplitude.
 13. The CPR apparatus of claim 12, furthercomprising a memory configured to store the sensed plunger displacementinformation.
 14. The CPR apparatus of claim 13, wherein the memory isfurther configured to store the plunger displacement information and oneor both of the current position of the reciprocating part of the plungerand the generated plunger driving instructions.
 15. A mechanicalcardio-pulmonary resuscitation (CPR) apparatus, comprising: a plunger; aplunger displacement sensor configured to sense plunger displacementinformation during a first set of reciprocating cycles; a microprocessorunit configured to: receive the sensed plunger displacement informationfrom the plunger displacement sensor; sample the received plungerdisplacement information for positional data of the plunger over one orseveral reciprocating cycles; determine one or both of a plunger drivingforce and a plunger displacement amplitude for one or severalreciprocating cycles based at least in part on the sampled plungerdisplacement information; and generate plunger driving instructions foradministering a second set of reciprocating cycles based at least inpart on the determined one or both of the plunger driving force and theplunger displacement amplitude for the one or several reciprocatingcycles.
 16. The CPR apparatus of claim 15, wherein the plunger ismechanically-controlled.
 17. A mechanical cardio-pulmonary resuscitation(CPR) apparatus, comprising: a plunger; a plunger displacement sensorconfigured to sense plunger displacement information duringreciprocating cycles and the plunger displacement sensor configured tosense a current position of the plunger; a microprocessor unitconfigured to: receive the sensed plunger displacement information andthe sensed current position of the plunger from the plunger displacementsensor; sample the received plunger displacement information and thesensed current position of the plunger for positional data of theplunger over one or several reciprocating cycles; determine one or bothof a plunger driving force and a plunger displacement amplitude for oneor several reciprocating cycles based at least in part on the sampledplunger displacement information and the sensed current position of theplunger; and generate plunger driving instructions based at least inpart on the determined one or both of the plunger driving force and theplunger displacement amplitude for the one or several reciprocatingcycles.